Some commercially available thermal fuses have limited temperature capability. These thermal fuses use a solder that is alloyed to melt at a desired trip temperature. The solder is suspended between two points in a circuit (bridge). The solder “bridge” melts and falls away at the trip temperature, thereby opening the circuit (fuse). Other thermal fuses use the same solder, but contain a spring and contact bar. When the solder reaches its melting temperature, the spring pushes the bar away from the contacts thereby opening the circuit. Thus, solder fuses are not resetable, which is important for many applications. However, there is potential for the solder bridge to migrate back into place under vibration or changes in unit orientation, causing a re-closure of the switch to occur. Also, solder-type thermal fuses have a limited temperature range due to the melting point of the alloyed solder.
Bimetallic thermal switches can be designed to trip over a range of temperature much greater than solder-type fuses. The setpoint for a bimetallic thermal switch is based on the type of bimetallic material used and the forming process of the bimetallic material. Although bimetallic switches can be produced to trip over a great range of temperatures, they are resetable. Bimetallic thermal switches toggle back to the “On” position (closed contacts) when the temperature drops below the trip value. However, many applications require that the thermal switch stays open even if the temperature returns to normal.
Therefore, there is an unmet need for unresetable thermal switches that can be used over a wide temperatures range.